Drought in California, November 1 to 30, 2015

Area Forecast Discussion
National Weather Service Sacramento CA
1015 PM PDT SAT OCT 31 2015

.SYNOPSIS...
Cool and wet weather starts Sunday as a Pacific storm
system moves into the West Coast. It will bring the first
significant snow event of the season to the high Sierra with
widespread rain elsewhere.

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.DISCUSSION...
Record warm Halloween at DTS (86 which ties record set in 1966)...
SCK (85 breaks old record of 83 in 1966)and MOD (83 which ties old
record in 1967) as high pressure stubbornly refused to release its
influence over Norcal. But alas...the wx is beginning to change as
satellite imagery shows cloud cover advancing over Norcal north of
the I-80 corridor already at press time...and radar detecting rain
moving inland across ORE. This area of rain is associated with a
cold front and PTW plume over 150% of normal that is forecast to sag
Swd into our CWA on Sun...roughly to the I-80 corridor around 3 to 5
PM. With the first short wave energy mainly passing through the
PacNW on Sun...rainfall amounts will generally be light with
orographic effects benefiting the Siernev. This will be more of a
WAA scenario as snow levels should remain above the major passes
with WBZs above 7 kft over the Nrn mtns and 9-10 kft over the
Siernev. It is the secondary short wave energy from the GOA that
will be digging SEwd into Norcal...along with the upper level jet...
late Sun nite/early Mon morning that will bring colder and more
unstable air and better dynamics for more robust precip in Norcal
generally after the weekend is over. Snow levels should lower below
the major trans-Siernev passes between 12z-18z Mon as suggested by
the WBZ progs. With the narrowing and eroding TPW plume shunted Swd
by the digging trof on Mon large scale ascent is also forecast to
weaken later in the day and precip should decrease/end from N to S
later in the day...although the Siernev will likely continue to see
some wrap-around showers Mon nite.      JHM

.Previous Discussion...

The forecast is on track for the first significant winter storm
of the season. A cold trough from the Gulf of Alaska will combine
with abundant moisture from an atmospheric river to bring
widespread rain late Sunday night through Monday. Models have been
trending wetter with this system, and are showing an enhanced band of
heavier precipitation over the Northern Sierra Nevada. The current
storm total projects a liquid equivalent of up to 3.0 inches in
that area. In terms of snow, this could be up to a foot
accumulating above 7000 feet.

Rain will gradually spread from north to south over the area
during the day Sunday. Snow levels will start around 9000 feet,
then dropping down to around 5000 feet on Monday. The major question
is when the snow levels will drop to 7500 feet, around the Donner
Pass level on I80 and Echo Summit on Highway 50. If it takes until
daybreak Monday, much of the precipitation may end up in the form
of rain in those areas. High mountain peaks will likely see a foot
or even more of snow. The peak snow intensity is expected Monday
morning. More showery precipitation is expected by afternoon, with
the potential for afternoon thunderstorms. Snow showers may drop
to around 5000 feet, though accumulations to those levels are
expected to be light. Showers may linger over the mountains
through Tuesday into early Wednesday.

A major concern is recently burned areas (past 3 years),
especially on the west slope of the northern Sierra Nevada
Mountains, such as the Butte Fire. The main band of precipitation
could drop 1 to 3 inches on these locations. Thunderstorms
developing in the afternoon could add additional locally heavy
rain to this, though when and where these could be remains
uncertain. People living and working in those areas should be
aware of the potential for debris slides on Monday. Stayed tuned
for updates to the forecast.

Valley locations could see around 0.25 to 0.50 inches of rain,
potentially more in convection. Drivers on Monday should be aware
of slick roads, especially if wet leaves are in the area. EK

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.EXTENDED DISCUSSION (Wednesday THROUGH Saturday)

Trough axis will continue to shift eastward on Wednesday, with
some lingering showers mainly over higher elevations. Breezy
northerly winds will be possible on Wednesday, mainly over the
Coastal range and the Western Sacramento Valley, as an upper high
pressure begins to build in over the west coast. High temperatures
will warm up again by next Friday to 5 to 10 degrees above normal.
Dry conditions will persist through the remainder of the forecast
period. However, the GFS suggests a short wave moving through next
Saturday, which could bring some Sierra showers. Have kept
forecast dry for now due to model uncertainties. Next chance of
rain might return by next weekend as an upper trough approaches
the Pacific Northwest. IDM

&&

.AVIATION...

Increasing clouds through the period with VFR conditions thru
early Sunday morning. Weather system will approach the area from
the NW Sunday with precip reaching northern Sac Valley by 12-15Z
and to the I-80 corridor toward the end of the period. Periods of
MVFR and possibly IFR conditions with precipitation, especially in
heavier showers. Winds generally below 10 kts at TAF sites with
breezier conditions across the Delta and higher terrain.
&&

.STO Watches/Warnings/Advisories...
Winter Storm Watch from Sunday evening through Monday evening
for West Slope Northern Sierra Nevada.

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